altering the php executable so that dangerous functions will be harmless, then you can safely eval the code. You need to be very certain that he the php executable is safe to execute. Don't forget these scripts are often evil.

using online script decoders like ddecode.com

The following techniques are frequently being used for executing obfuscated scripts.

The ROT13 encoding simply shifts every letter by 13 places in the alphabet while leaving non-alpha characters untouched. Encoding and decoding are done by the same function, passing an encoded string as argument will return the original version.

string rawurldecode ( string $str )
Returns a string in which the sequences with percent (%) signs followed by two hex digits have been replaced with literal characters.